This invention relates generally to ground fault measurement. More particularly, this invention relates to an algorithm to prevent nuisance tripping due to inaccurate ground fault current calculation.
Ground faults are the result of currents flowing through a ground return path. In electrical systems, current should flow from one of the power phases out to the electrical load and return on another power phase or the neutral circuit. The detection of ground fault currents is a common protective mode for high function circuit breakers. The manner that this protection is accomplished varies, but a common approach is to calculate the ground fault current by summing the values of phase and neutral, if in use, currents. In normal, unfaulted use, this signal is zero. If a signal exists, it indicates that some of the current is returning via a ground path. Then, this measured ground fault current is compared to an operator-selected threshold to determine if the circuit breaker should trip.
High level fault currents provide one common source of inaccuracy in a ground fault measurement. When high fault or temporary application currents (for example, motor starting) flow, the small errors due to tolerance or sensor saturation can become relatively large when compared to the ground fault signals that a trip unit attempts to detect. Due to the short trip times customary in ground fault protection, these errors can cause false or nuisance tripping.
A nuisance trip in a circuit breaker can cause expensive processes to shut down. It can have serious ramifications and be very costly. Ground fault protection can be a source of nuisance tripping if the method for determining the ground fault current is, under certain conditions, sufficiently inaccurate.
Inaccurate ground fault current calculations occur when the threshold for ground fault protection is very small compared to the measured phase currents. If the phase currents are sufficiently large and the ground fault threshold is sufficiently small, the error in measuring ground fault current can result in a level that can exceed the specified trip level for a ground fault circuit.
One method used to deal with the inaccuracy noted above is to make the measurement of the phase currents so accurate that the calculated ground fault current is sufficiently accurate. This is difficult and expensive to do especially when large phase currents are used to calculate small ground fault currents. Another method involves eliminating or disabling the ground fault function when fault currents exceed a specified level. This avoids the nuisance tripping but may allow incremental damage in the faulted circuit (due to a longer tripping time), particularly if the high level fault is entirely a ground fault.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a trip unit including phase protection circuits, a ground fault protection circuit, and a phase current qualification circuit intermediate the phase protection circuits and the ground fault protection circuit, wherein the phase current qualification circuit includes circuitry for determining a minimum ground fault setting for detected phase currents.
In another exemplary embodiment of the invention, a trip unit includes means for determining a minimum ground fault setting for detected phase currents, means for comparing the minimum ground fault setting to a customer provided threshold, means for determining which of the minimum ground fault setting and the customer provided threshold has a larger value and setting the larger value as a maximum value, and means for utilizing the maximum value as a ground fault threshold.
In another exemplary embodiment of the invention, a circuit breaker includes a plurality of phases, a current transformer for each phase, phase protection circuits receiving signals from each current transformer, a ground fault protection circuit receiving signals from each current transformer, and a phase current qualification circuit intermediate the phase protection circuits and the ground fault protection circuit, wherein the phase current qualification circuit includes circuitry for determining a minimum ground fault setting for detected phase currents.